Planar shaped articles, in particular plates of brittle materials, preferably those having a low coefficient of thermal expansion, such as glass or glass-ceramic plates, are used in many industrial and domestic fields. Typical examples are insulating window panes, hot plates having a glass-ceramic plate and glass shelves in refrigeration appliances.
To facilitate handling of such planar shaped articles, in particular during mounting, and in order to avoid a risk of breakage emanating from the edge regions, the shaped articles are placed in a holding frame. However, these holding frames typically consist of materials which have a different coefficient of thermal expansion from the shaped-article material.
Typical values of the coefficient of thermal expansion are listed below.
______________________________________ Shaped-article material Glass-ceramic, e.g. the material .alpha.&lt;0.5 .multidot. 10.sup.-6 1/K known under the trade name Ceran .RTM. Soda-lime glass .alpha..about.9 .multidot. 10.sup.-6 1/K Borosilicate glass, e.g. the .alpha..about.3.3 .multidot. 10.sup.-6 1/K material known under the trade name Duran .RTM. Frame materials Aluminium .alpha..about.25 .multidot. 10.sup.-6 1/K Steel .alpha..about.12 .multidot. 10.sup.-6 1/K Plastics (general) .alpha. typically 80 . . . 200 .multidot. 10.sup.-6 1/K Special plastic, e.g. the glass- .alpha..about.15 .multidot. 10.sup.-6 1/K fibre-reinforced polyester known under the trade name Nu-Stone from Industrial Dielectrics Inc. ______________________________________
This difference in the coefficients of thermal expansion is mechanically particularly noticeable precisely in applications in which the shaped article, in particular plate, which has been placed in a holding frame is subject to severe temperature fluctuations, e.g. in the case of a glass-ceramic cooking surface, and particular consideration has to be given to this difference in terms of design and production when encapsulating the plate in the frame. Further factors are that it is necessary, as a result of the different expansions, in particular in the case of glass-ceramic cooking surfaces, to ensure a permanently sealed connection between plate and frame throughout the service life and for the cooking surface to be mounted in a permanently elastic manner under mechanical impact loading.
It is known to fix plates made of glass, glass-ceramic or other brittle materials in frames made of plastic or metal by means of permanently elastic adhesives (e.g. silicone adhesives and the like). Examples are window frames or glass-ceramic cooking surfaces. The permanently elastic adhesive in this case compensates for the different thermal expansions while retaining the seal.
Furthermore, so-called dry-mounting methods are known, in which the glass or glass-ceramic plate is clamped between permanently elastic sealing layers. The permanently elastic intermediate layers are required, on the one hand, so that it is not possible for any direct contact, which would lead to the surface being damaged and thus to breakage, to occur between the brittle material and, for example, the metal frame under mechanical stressing. On the other hand, this intermediate layer compensates differences in planarity and differences in the thermal expansion. The latter is extremely important, particularly for glass-ceramic cooking surfaces, since the glass-ceramic exhibits virtually no expansion at high temperatures, but the frame, depending on the material, generally does expand at elevated temperatures. In addition, the elastic intermediate layer largely avoids stress being transmitted from the frame onto the brittle glass/glass-ceramic material.
It is known from DE-U-90 04 180.1 to design shelves of a domestic refrigerator as glass plates which are encapsulated by a closed plastic frame, the plastic frame being formed in a simple manner by encapsulating the glass plate by injection moulding. This encapsulation by injection moulding is carried out by means of known, simple injection moulding processes, in which plastics are injection-moulded directly around panes of glass. In this process, the pane of glass is placed in a mould, into which molten plastic is then injected at high pressure. The plastic, which flows in the plastic state, in this case fills the cavities of the mould and encloses the pane of glass in a sealing manner, provided that the frame engages over the pane of glass sufficiently. However, a problem with this process is that the plastic shrinks to a relatively large extent, and as a result relatively high curving stresses are induced in the pane of glass. Moreover, in practice, this application is hindered by differences in the thermal expansion. On the other hand, it is possible, for example by tempering the glass, to make the latter able to withstand substantially greater mechanical loads, so that it does not break when curving stresses occur. An application of this kind is described in U.S. Pat. No. 5,362,145, in which tempered soda-lime glass is encapsulated with a synthetic polypropylene material by injection moulding.
Such curving stresses are reinforced, primarily, by the frame design as well, if, for example during cooling, an additional force component arises in the direction perpendicular to the plane of the plate as a result of non-uniform shrinkage. Particularly in the case of thin plates, the tendency to curving is more pronounced than in the case of a relatively thick plate. Furthermore, this behaviour is adversely intensified if the sides of the glass plate have different surface structures (protrusions, printing, etc.), which additionally results in a different strength level. Therefore, even minute damage to the surface can lead to breakage.
Particularly when the brittle material glass-ceramic for the shaped article is combined, in the form of a plate, with plastics as the frame material, this may lead to problems in manufacture and over the service life, because of the lower mechanical strength of the plate compared to thermally tempered glass and because of the very different thermal expansions at severely fluctuating temperatures, such as for example in the case of a cooking surface. A mounting profile strip for glass-ceramic cooking hobs which has a flange section enclosing the glass-ceramic plate in the manner of a frame is known from EP-0,391,122 A2. This document also provides for injection-moulding the frame, which is formed from the mounting profile strip, as a single-piece component directly onto the edge of the glass-ceramic plate in one operation and without seams, so that it is in direct contact with the said glass-ceramic plate.
Due to the direct contact of the glass-ceramic plate with the injection-moulded plastic frame encapsulating it, for the reasons described above the brittle material is subjected to high mechanical loads during injection moulding, to say nothing of the problems during use.